Softwood and hardwood lumbers have long been used as structural components in the construction industry, due to their desirable strength characteristics, relatively low cost and ease of manufacture and working. As the cost of lumber has increased, alternatives such as hardboard have been selected, due to their lower cost. Hardboard usually consists of a cellulose fiber, water and a binder such as latex, starch or urea formaldehyde. However, all of these alternatives tend to suffer from depletion, low strength-to-weight ratio, or the use of undesirable solvents or binders in their manufacture or processing.
For many years, corrugated fiberboard has served as a basic, light weight material for packaging and other light duty applications. Corrugated fiberboard is made from flat fiberboard material. A single sheet is corrugated to form the middle core, or corrugated medium. This requires a separate operation. Adhesive is then applied to the nodes of either one or both sides of the corrugated middle core, and then bonded to one or two flat sheets, respectively. The shape of the core is maintained by the bonds. However, panels of corrugated fiberboard are relatively weak, and do not lend themselves to structural applications.
It has also been known to produce certain pulp molded articles, such as egg cartons, pots for flowers, baskets, and the like. These products are made on rigid molds. The mold is often semi-porous and is covered with screening material. A vacuum is pulled at the back of the mold, causing flow through the screen and the mold so that the fibers form a uniform mat over the screen. The mat on this rigid mold is consolidated with a mating reverse shaped solid mold pressed against the mat on the forming mold. This consolidates the mat between the two mating molds. The direction of the consolidation force is perpendicular to the mat. However, such articles lack the strength to be useful as structural components.
To increase the strength of formed panels, it has previously been known to produce stressed-skin panels for use as structural components. Such panels are considered advantageous due to their high strength-to-weight ratio. However, the high cost of fabrication has limited the commercial practicality of such components to high cost, exotic applications.
Previously, such panels were constructed in layers, wherein a skin layer is affixed to an internal grid, commonly called a "honeycomb." These honeycombs are often fabricated from flat sheets or strips of paper or paper-like materials, which are combined by means of spaced spots of glue. The assemblage is pressed and then the adhesive is allowed time to cure. A second skin is often applied in a similar manner, and the assemblage is trimmed to the desired dimensions.
Prior methods of forming stressed-skin structural panels are found in U.S. Pat. Nos. 4,702,870, 4,753,713, 5,198,236, 5,277,854, and 5,314,654.
More recently, the basic components of stressed-skin panels have been molded from various fiber materials. Two such components are then glued together to form the complete panel. See, for example, U.S. Pat. No. 4,702,870. Panels constructed in this manner provide advantages over the prior art, namely, the avoidance of a number of gluing and other fabrication steps, and the greater flexibility of production of many different sorts of open cell grids.
However, the finished panels produced in accordance with the prior art generally require at least one gluing step, and the attendant manipulations, thus adding to the cost. Furthermore, the surface area available to be utilized as a contact area for the glue is generally rather small, so that minor misalignments between the two layers would substantially lessen the strength of the glue bond.
This prior art also embodies fundamental limitations on the production of such panels and on the panels so produced. The mold inserts used in the prior art to form the internal grid suffered from compression set, whereby repeated use changed the shape of the inserts. This leads to a) poor grid formation, producing weaker and inconsistent products; b) interference with the product panel in the mold, producing poor release and possible product damage and requiring use of mold release agents and manpower that increases costs and also diminishes product quality; c) increasing production time, thereby increasing costs and severely limiting the ability of such products to compete with other, less expensive materials; and d) limitations on the raw materials that would be useful.
In the present invention it has been discovered that new mold insert shapes and arrangements avoid these disadvantages and produce superior panels under production techniques that open the door for such panels to compete with a wider range of current products.